The valves in the foreground are part of Thames Water's operational site. The buildings are all museum. The building on the right is a 1908 extension that housed a Worthington non-rotative engine that was replaced by electric motors. The middle building is 1837/8 and houses the Boulton & Watt, Maudslay and Bull engines. The far building is called The Great Engine house and was built in two phases - 1845-6 and 1869-70. It houses the 90" and 100" Cornish engines.
TQ1878 : Inside Kew Bridge Steam Museum's Tower Magnificent photo. Know the site very well and have crawled around all sorts of holes in the ground where the public are not allowed. You are right about the tower - it is a slog but the view is worth it.